1. Field of the Invention
The present invention generally concerns the cooling of electrical coils. The present invention in particular concerns a cooling method for better heat dissipation at gradient coils and shim systems of magnetic resonance tomography apparatuses.
2. Description of the Prior Art
Electrical coils generally possess a power or stability limit that is defined by the limited dissipation of the heat due to ohmic loss. Such coils are used in magnetic resonance tomography (MRT), for example in the form of gradient coils and shim coils.
Gradient coils serve for the spatial coding inside an MRT apparatus by generating a three-dimensional orthogonal gradient field that is superimposed on the static homogeneous basic magnetic field in the x-direction, y-direction and z-direction. The x-coil and y-coil are typically a coil type known as saddle coils that are rotated with respect to one another by 90° with regard to the z-axis. The z-coil represents a Maxwell coil.
An exact image reconstruction in MRT is only possible when, during the measurement, the gradient coils exhibit a sufficient temporal magnetic field stability and the static basic magnetic field is sufficiently homogeneous.
Among other things, two techniques are known for homogenization of the basic field magnets:
1. A further orthogonal coil system with current flowing through it is located within the orthogonal gradient system, with which it is possible to homogenize the basic field magnet. These additional correction coils (also called shim coils) serve to compensate field inhomogeneities of higher orders and are designed in a very complicated manner in that they are interwoven with the gradient coils.
2. For further homogenization of the basic magnetic field, a suitable arrangement of paramagnetic bodies (shim irons) that are integrated into the gradient coil is calculated with the aid of a field calculation program. The curve of the magnetic field lines of the base field and of the gradient fields can be influenced by the size and position of the shim irons. An advance measurement of the field distribution serves as a basis for the calculation. Another control measurement is conducted after the mounting. This process must be repeated multiple times before a satisfactory shim result is achieved. The shim irons typically are introduced into drawers axially in openings known as shim channels in the tube wall of the gradient system. In order to avoid or to minimize eddy currents in the shim irons, the respective shim iron blocks (composed of playing card-sized shim plates) are stacked.
The technique under point 1 is known as active shimming, the technique under point 2 is known as passive shimming. The combination of both techniques is known as a shim system.
The purpose of the gradient coil current supply and shim coil current supply is to generate current pulses of precise amplitude and at precise times, corresponding to the measurement sequence used. The required currents are approximately 250 amperes, the current rise rate is in the range of 250 kA/s.
Under such conditions a large amount of heat arises in the gradient coils and in the shim coils due to electrical power loss on the order of approximately 20 kW, which heat must be actively dissipated in order to prevent the electromagnetic behavior of the gradient and shim system (and therewith the imaging itself) from being impaired.
A heating of the shim irons (due in part to ohmic losses of eddy currents that cannot be avoided, due in part to heat transfer of the gradient and shim coil heat via the sealing material) cannot be avoided and would significantly impair the shimming if the shim irons were not cooled. However, the heating of the shim irons is smaller by orders of magnitude (approximately 5 W) than that of the gradient coils and shim coils, which is why an elaborate active cooling of the individual shim irons is not absolutely necessary.
According to the prior art, the cooling of convention electrical coils as well as the cooling of gradient coils, shim coils and shim irons in magnetic resonance tomography ensues either by air surface cooling (air blown past) or by water cooling. Active water cooling has conventionally represented the most efficient cooling. The heat is typically transferred from the conductors to be cooled into heat-dissipating flowing water via more or less poorly-conductive plastic layers. The resulting resistance to heat flow limits the maximal capacity of the water cooling.